A filament comprises at least one emitter. An emitter is a component that releases energy, as in the form of electrons, upon the absorption of energy. In the filament, the emitter is one element and the filament can include additional features. Alternatively, the filament can comprise a plurality of emitters.
Conventional filament designs for lighting and electronic emission generally comprise a helical coil geometry. While a helical coil has proven adequate for many applications that require relatively isotropic illumination, a helical coil may be inefficient for electronic emission. This inefficiency is partly due to space-charge limitations on emission current, which result in low saturation, and hence a weak signal. Additionally, a large fraction of electron trajectories reaches an associated anode outside a desired target area, leading to an undesirable focal spot profile.
The prior art in filaments, emitters, filament manufacture and support assemblies focuses on tungsten helical coil emitters. Attachment of helical coil filaments to supports is accomplished by crimping the filament wire inside electrically conducting leads. The techniques used in this method of attachment often result in filament misalignment, leading to undesirable focal spot characteristics.
Ribbon-like filaments, and their emitters, have been known in the art for illumination and electronic emission purposes. These ribbon filaments generally comprise a single emitter. These known ribbon filaments comprise integrally formed leads, and are thus difficult to attach to supports with a desired alignment accuracy. The integral-lead configuration compromises the filament alignment in a cathode assembly because the ribbon filaments are prone to warp as the integral leads are twisted during attachment to the support structure.
Near-isothermal heating is exhibited in sufficiently long helical coil filaments due to the coils possessing an extended length of uniform cross-section. The uniform cross-section results in essentially negligible heat conduction along a potion of the filament. Known ribbon filaments do not maintain a uniform temperature across the emitter and hence do not approach their potential thermionic emission current or life. Further, known ribbon filaments do not possess an engineered temperature distribution across the filament, and thus do not achieve their potential focal spot quality. Further deficiencies of known ribbon filaments include inadequate mounted stability and ease of alignment with a support and mounting structure,
It is therefore desirable to improve performance of filaments and associated emitters by introducing filament designs that produce desired temperature distributions across emitters and prolonged emitter life, while attaining high emission currents and good focal spot quality. Also, it is desirable to provide filament geometries that offer substantial mounting advantages over conventional helical coils. The mounting advantages include, but are not limited to, enhanced focusability, geometric stability, consequent durability and ease of alignment within a filament mounting structure, and retention of focal spot quality.